1. Field of the Invention
The present invention relates to a multi-angle colorimeter for measuring a gonio-apparent coating such as a metallic coating or a pearlescent coating, and more particularly, to a multi-angle colorimeter having a function for correcting the geometry errors.
2. Description of the Related Art
In metallic coatings and pearlescent coatings used primarily for an automotive coating, tiny flakes of aluminum or mica called special effect pigment are contained within the coating, and this produces a so-called metallic effect or pearlescent effect. This is due to the fact that the contribution of the special effect pigment to the reflection characteristic differs according to the illumination and the viewing direction. For the evaluation (calorimetric measurement) of such coatings, a multi-angle colorimeter that has a multi-angle geometry (optical system) is used. A multi-angle colorimeter having the multi-angle geometry illuminates from a plurality of directions and receives reflected light from a single direction or illuminates from a single direction and receives reflected light from a plurality of directions.
However, generally, the following two geometry errors are inevitable in the multi-angle colorimeter of prior arts:
(1) Initial directional error: This error is caused because the angles from the normal of the sample surface to be measured (anormal angles, hereafter) of the illumination and the receiving are deviated from the nominal angles due to various manufacturing errors.
(2) Orientation error: This error is caused by the disagreement between the normal of the sample surface and the reference axis of the measuring device both of which are to be in coincidence ideally. This error readily occurs when the sample surface is not a plane.
These geometry errors significantly affect the measurement stability particularly in the vicinity of the specular reflection direction where the directional dependence of the reflection characteristics is very high. For example, in a multi-angle colorimeter illuminating from a direction of 45 degrees in anormal angle and receives reflected light from five directions of 15 degrees, 25 degrees, 45 degrees, 75 degrees and 110 degrees in angles from the specular reflection direction (aspecular angles, hereafter), the influence of the geometry errors on measurement values is particularly large in directions of 15 degrees and 25 degrees in aspecular angle.
To reduce the influence of the orientation error, a correcting technique of the orientation error has been proposed. According to this method, the influence of the orientation error is estimated based on a mathematical function approximating the direction dependence of the reflection characteristic of the sample illuminated from the direction of 45 degrees in anormal angle, and then corrected based on the estimated error.
However, the above-described method doesn't correct the influence of the orientation error accurately since the illumination from the direction of 45 degrees anormally in typical multi-angle colorimeters is used as the illumination for detecting the orientation error. That is, the reflection characteristic is not symmetrical in both sides of the specular direction for the illumination from 45 degrees in anormal angle largely away from the normal of the sample surface because of the asymmetrical optical characteristics of the sample. For this reason, the approximation accuracy of the mathematical function is generally insufficient, and accordingly the accuracy of the orientation error estimated based on the approximation function is also insufficient.